The present invention relates to improvements in a line pressure control apparatus of an automatic transmission which apparatus controls a line pressure of the automatic transmission under a lockup condition so as to ensure an engagement capacity and an amount of lubrication oil for an engaged friction element under a high vehicle speed condition.
Japanese Patent Provisional Publication No. 2-309056 discloses a conventional line pressure control apparatus of an automatic transmission which apparatus is arranged to lower the line pressure when an engagement of a lockup clutch of a torque converter is established. More particularly, during the time period from the output of the lockup engagement command to the time elapsed the predetermined time period, the line pressure is gradually lowered according to an input torque of the automatic transmission. Further, when the lockup engagement is accomplished, the line pressure is set at a value lower than that of the normal condition. In this conventional line pressure control apparatus, the balance between the supply and consumption of lubrication oil at the automatic transmission under the lockup condition is controlled by decreasing the leak amount of lubrication oil for the automatic transmission. The decrease of the leak amount is executed by setting the lockup line pressure at a lower value as compared with that of the normal condition.
In contrast, the necessary amount of the lubrication oil in the automatic transmission is generally determined according to the input torque to the engaged friction element such as the engine torque and the rotation speed of the engaged friction element such as the vehicle speed. However, the conventional line pressure control apparatus is arranged to determine the amount of the lubrication oil for the automatic transmission only by the engine torque. This may cause a shortage of the lubrication oil when the vehicle travels in a high vehicle speed.